The Rio Grande aquifer system in westernmost Texas corresponds to the eastern part of the Southwest alluvial basins aquifer system, which is a large system of aquifers in alluvial basins in the southwestern United States and Mexico. These aquifers were studied as part of the U.S. Geological Survey's Regional Aquifer-System Analysis program and are discussed in detail in Chapter C of this Atlas. A brief description and discussion of the aquifers as they exist in Texas are presented here. The Rio Grande aquifer system in Texas is in Culberson, El Paso, Hudspeth, Jeff Davis, and Presidio Counties. The alluvial aquifers are found in six major basins: the Mesilla, the Hueco, the Salt, the Eagle, the Red Light, and the Presidio. The Rio Grande aquifer system is in the Basin and Range Physiographic Province. Vertical movement along block faults has resulted in structurally high mountain ranges that trend south and southeast and are separated by structurally low parallel basins. The basin a